One searches for a word and the site produces a quick definition and links to multiple sites that define the word. One can jump around from site to site - as some are better or more complete than others. And it will break definitions down into categories like Art or Music or Science so one can focus on those if one is looking for a particular definition.

Many of these sites also have audio pronunciations and if I want to hear a word, I just keep clicking till I find one.

I like the wide assortment of definitions but the url is easy to remember so if I'm working with someone else and we want to look up a word, it's easy to direct them to the site.